This invention relates to microwave semiconductor device amplifiers, and more particularly to broadband avalanche diode amplifiers realized in microstrip form.
Microstrip Impatt (impact avalanche transit time) diode amplifiers as presently understood may incorporate a one-section quarter wavelength microstrip transmission line transformer to match the impedance of the output line to the diode impedance. Typically, the diode impedance is about 5 ohms while the characteristic impedance of the output line is commonly 50 ohms. Although it is known that a one-section transformer is frequency sensitive with the result that the Impatt amplifier has narrow band characteristics, the problem with the use of a broadband two-section quarter wavelength transformer is that spurious modes are observed, in particular the so-called "half-wave" resonance of the odd mode. The width dimension of the one-section transformer is less than one-half wavelength at the source frequency assuming a practical minimum value of the thickness of the microstrip dielectric substrate, so that the undesired half-wave odd mode resonance is inhibited. For the same source frequency and substrate thickness, the width of the wide section of a two-section transformer approaches or exceeds a half wavelength, with the result that occurrence of the spurious mode resonance has prevented its use. It is evident, of course, that broadband microwave semiconductor device amplifiers are needed for many applications including phased array radar and communication systems, and that microstrip circuits have the advantages of low cost and ease of fabrication.